There is great current interest in a light-emitting diode, whose raw material is a III-V group compound semiconductor (for example, AlGaAs, AlGaInP and AlGaInN), as an energy saving and long-lifetime light source for lighting devices in the next generation which will be used in place of an incandescent lamp and a fluorescent lamp.
The light-emitting efficiency of a light-emitting diode is generally determined by the product of the internal quantum efficiency and the efficiency of extraction of light to the outside space. With regard to internal quantum efficiency, particularly that in a system of AlGaAs or AlGaInP-series material, almost 100% of the internal quantum efficiency has been realized by the recent progress in crystal growth technologies. On the other hand, it is very difficult to efficiently extract the light generated in the active layer to the outside space. It is not too much to say that this is the largest factor limiting the light-emitting efficiency of a light-emitting diode.
Two fundamental reasons are responsible for that problem. A first reason resides in the problem concerning the so-called total internal reflection of light. Specifically, the refractive index of a III-V group compound semiconductor is generally considerably larger than 1, and a large part of the light to be emitted is totally reflected at the interface between the semiconductor and the air and is returned back into the semiconductor. Light which can be extracted from the system to the outside space is one incident to the interface at an angle smaller than the critical angle of total internal reflection. In the case of, for example, GaAs, the critical angle of total internal reflection is about 16.1°. The ratio of the light whose critical angle is within this value is only: 0.5×(1−cos 16.1°)=2%. Moreover, a part of the light within the critical angle is reflected on the surface, and the light that can be extracted from the system to the outside space is indeed only about 1%. In order to suppress the total internal reflection of light, hitherto, the below-mentioned methods, for example, have been adopted: (1) a method in which a resin having a high refractive index is used to encapsulate; (2) a method in which the crystal is formed into an inverse pyramid shape by dicing {M. R. Krames, M. Ochiai-Holcomb, G. E. Hofler, C. Carter-Coman, E. I. Chen, I. -H. Tan, P. Grillot, N. F. Gardner, H. C. Chui, J. -W. Huang, S. A. Stockman, F. A. Kish, M. G. Craford, T. S. Tan, C. P. Kocot, M. Hueschen, J. Posselt, B. Loh, G. Sasser, and D. Collins, “High-power truncated-inverted-pyramid (AlxGa1-x)0.5In0.5P/GaP light-emitting diodes exhibiting >50% external quantum efficiency”, Applied Physics Letters, 75 (1999) 2365-2367.}; (3) a micro-cavity is used to control the radiation of light {H. Benisty, H. DeNeve, and C. Weisbuch, “Impact of planar microcavity effects on light extraction-part I: Basic concepts and analytical trends”, IEEE Journal of Quantum Electronics, 34 (1998) 1612-1631.}; and (4) fine roughness is intentionally formed on the surface of a semiconductor, to change the incident angle of light, by utilizing the scattering of light at the interface (R. Windisch, B. Dutta, M. Kuijk, A. Knobloch, S. Meinlschmidt, S. Schoberth, P. Kiesel, G. Borghs, G. H. Dohler, and P. Heremans, “40% Efficient Thin-Film Surface-Textured Light-Emitting Diodes by Optimization of Natural Lithography”, IEEE Transactions on Electron Devices, 47 (2000) 1492-1498.}.
A second reason resides in the blocking of light by an opaque metal electrode. The driving current of a light-emitting diode is usually injected from a metal electrode formed on the surface of a crystal. In this case, though the density of current just under the electrode is the highest of all, the light generated there is almost blocked by the electrode. Therefore, the light is not allowed to escape to the outside space. In order to solve this problem, some techniques have been proposed which are developed by focusing on diffusion of the injected current to the outside area of an electrode region in the lateral direction as much as possible. Typical techniques include: (1) a method in which a thick current-spreading layer (about 10 μm) is provided between an active layer and an electrode {H. Sugawara, M. Ishikawa, and G. Hatakoshi, “High-efficiency InGaAlP/GaAs visible light-emitting diodes”, Applied Physics Letters, 58 (1991) 1010-1012.}; (2) a method in which a composite structure of a metal electrode and an ITO transparent electrode is used {D. J. Lawrence, D. C. Abbas, D. J. Phelps, and F. T. J. Smith “GaAs0.6P0.4 LED's with efficient transparent contacts for spatially uniform light emission”, IEEE Transactions on Electron Devices, ED-30 (1983) 580-585; J. -F. Lin, M. -C. Wu, M. -J. Jou, C. -M. Chang, B. -J. Lee, and Y. -T. Tsai, “Highly reliable operation of indium tin oxide AlGaInP orange light-emitting diodes”, Electronics Letters, 30 (1994) 1793-1794.}; and (3) a method in which a current-blocking layer is introduced into a place just under the electrode {H. Sugawara, K. Itaya, H. Nozaki, and G. Hatakoshi, “High-brightness InGaAlP green light-emitting diodes”, Applied Physics Letters, 61 (1992) 1775-1777.}. However, even if any of these structures is adopted, the situation where the density of current just under the metal electrode or in the vicinity thereof is the highest of all is not changed, and the effects of these structures are limited. Further, these structures give rise to the problem that the introduction of the thick current-spreading layer or current-blocking layer complicates production processes of such the devices and raises production cost.
Moreover, in each structure of the conventional light-emitting diodes, carriers injected from an electrode are concentrated in the vicinity of the electrode and the density of carriers, i.e. luminescence intensity, sharply decreases at a place apart from the electrode. Thus, there is a limitation to the output power of light obtained from one light-emitting diode chip. In the case of applications requiring high output light power, it is necessary to use a large number of chips.